Of the major building materials, wood is the only one that is organic in origin. This accounts for much of its uniqueness as a material of construction. Trees grow naturally in the forest, and most of the work of ÒmanufacturingÓ wood is done for us by the solar-powered processes of growth within the tree itself. This makes wood a renewable resource and inexpensive to produce: Trees need only be harvested, cut to size, and dried before they are ready for use in construction. On the other hand, we have only limited control ofthe quality and properties of wood. Unlike steel, concrete, or masonry, we can do little to refine solid wood tosuit our needs. Rather, we must accept its natural strengths and limitations.

Wood is well suited to our building structures. The tree itself is a wooden structure, a tower, erected for the purpose of displaying leaves to the sun.* The tree is sub-ject to the same forces as the buildings we erect: It supports itself against the pull of gravity; it withstands the pressures of wind and the accumulation of snow and ice on its limbs; and it resists the natural stresses of the envi-ronment, such as changes in temperature and moisture, and attack by other organisms. Wood has evolved to per-form these jobs well in the tree. Not surprisingly, it also performs them well in our buildings.

In nature, the greatest forces acting on a tree, such as the pressure of the wind, tend to act in bursts of relatively short duration. In response, wood has evolved to be strongest when resisting forces that act brieß y rather than those that act over longer periods of time. When we build with wood, we recognize this unique property. In the engineering design of a wooden structure, the maximum loads that the structure may carry are increased as the length of time these loads are expected to act decreases.

This increase, called the load duration factor, ranges from 15 to 100 percent over the basic design strength of the material and is applied to wooden structures when considering the effects of wind, snow, earthquake, and impactÑ the same types of loads the tree has evolved to withstand most effectively.

The structural form of the tree is also well suited to its environment. Tree branches are supported at one end onlyÑan arrangement called a  cantilever. The joint sup-porting a cantilevered branch must be strong and stiff, efficiently utilizing the materials structural capabilities to the utmost. Yet despite the stiffness of thejoint, the branch it supports can deß ect relatively large amounts since it is fastened at one end only. Underheavy loads, such as wet snow, the branch can droop and shed its load. Likewise, under strong winds,the tree and its branches can sway and shed much of the force acting upon them.

The trees capacity to bend and sway helps it to survive forces that might otherwise break it. When we build with wood, we cannot directly exploit the efficient structural form of the tree. The large deß  ections characteristic of this form are unsuitable for buildings. They would cause discomfort to building occupants and place undue stress on other building components.

Nor can we exploit the naturally strong joints that join a trees limbs, despite their benefits in stiffness and economy of material. In preparing wood for our uses, we cut the tree into pieces of convenient size and shape, destroying the continuity between its parts. When we reassemble these pieces, we must devise new ways to join them. Despite the many methods developed for fabricating wood connections, joints withstrength and stiffness comparable to the trees are rarely achievable. Rather, we rely on much simpler types of connections that suffice when a beam is supported at both ends.

Wood in a tree performs many functions that wood in a building does not, because wood is involved with all aspects of the trees life processes and growth. In order to meet numerous and specialized tasks, wood has evolved a complex internal structure that is highly directional.

The fact that wood is anisotropic, that is, it has different properties in different directions, affects virtually all of its properties. The direction of the grain in a piece of wood influences its production, shaping, fastening, strength, durability, aging, and beauty. No other major building material, perhaps with the exception of some specialized reinforced concrete systems, has such directional characteristics.

Consider two wood building systems, the log cabin and the heavy timber frame. Log cabin construction is appropriate to the most primitive construction technology. Logs found on or near the site are prepared for use with a minimum of labor and simple tools. Once assembled, the logs provide structural support and act as interior finish, exterior cladding, and insulation. Though a practical solution to providing shelter, log cabins use wood inefficiently and result in a structure  that is dimensionally unstable due to the large stacks of  cross-grain lumber. Where more sophisticated tools and methods are available, heavy timber takes the place of log cabin construction. In this system, wood members are assembled in ways that more closely resemble the tree form. The resulting structure is stronger, requires less material, and is more stable. A simple change in the orientation of the wood members produces a completely different building system, one that is more efficient, durable, and comfortable.

Modern developments in the wood construction materials industry reß ect efforts to overcome the limitations of wood in its natural state and to make the most  efficient use of the material. Wood panels made of ply- wood are larger and more stable than those that can  be produced from solid boards while minimizing the
adverse effects of wood grain and natural defects. Panel  products such as OSB and hardboard make use of wood waste that might otherwise be discarded. Glue-laminated and composite lumber beams can be produced in sizes and of a quality exceeding those available in solid timbers. Chemical treatments can protect wood from fire or decay. Thus, the trend is toward using wood less in its natural state and more as a raw material in sophisticated manufacturing processes to produce more refined and higher-quality construction components. Yet despite such advances, the tree itself remains an inspiration in its grace and strength, as wellas in the lessons  it offers for our understanding of building materials and methods.

*Brayton F. Wilson and Robert R. Archer, ÒTree Design: Some  Biological Solutions to Mechanical Problems,Ó BioScience, Vol. 29, No. 5, p. 293, 1979.

0 comentarios:

Post a Comment